The invention relates to radiation dosimeters and specifically to those having a passive radiation sensor, which is able to keep a record of an absorbed radiation dose, without using a power source and without utilizing external recording mechanisms.
Radiation dosimeters have wide application to a variety of industries, including medical X-ray and nuclear imaging facilities, nuclear power plants, nuclear spent fuel disposal, homeland security radiation monitoring, and food irradiation systems.
US patent application publication US2015/0162369 A1, entitled “Single-poly floating gate solid state direct radiation sensor using STI dielectric and isolated Pwells”, discloses a passive radiation sensor utilizing floating-gate MOSFET (i.e. Metal Oxide Silicon Field Effect Transistor) technology. In such a sensor, the gate voltage of a pre-charged transistor decreases in response to radiation absorbed by the sensor. By means of pre-determined calibration curves, a measured decrease in gate voltage may be converted to radiation absorption, expressed in physical units of Grays, where one Gray equals one Joule of absorbed radiation energy per kilogram of exposed mass. In some applications, the radiation absorption may be further converted to a personal dose equivalent, expressed in units of Sieverts, which accounts for the relative health effects of different kinds of ionizing radiation and different incident particle energies. One Sievert is equivalent to the amount of radiation absorption needed to produce the same effect on living tissue as one Gray of high-penetration x-rays.
Floating-gate MOSFET radiation sensors may be subject to charge loss, or gain, resulting from mechanisms of different nature, such as thermal stress, which occur in the absence of incident radiation and which reduce the gate voltage of the MOSFET. This phenomenon, known as “voltage retention loss”, depends upon circuit design as well as various ambient factors, such as temperature of the sensor during operation. Voltage retention loss, if uncompensated, will produce false readings of the radiation absorption.
Various methods of accounting for the effect of voltage retention loss have been reported in the literature. For example, U.S. Pat. No. 6,172,368 issued to Tarr, on Jan. 9, 2001, discloses the use of two radiation-sensitive floating-gate transistors, preferably having charges of opposite polarity, and measuring the difference between the threshold voltages of the two transistors. One difficulty with this approach is that both transistors must be matched in their sensitivity to absorbed radiation over a wide range of incident particle energies and must be matched in their voltage retention loss over a wide range of ambient temperatures. This is difficult to achieve in practice.